Although it wouldn't happen on an organic level, there are a few cards that let you cast sorcery cards at instant speed. In the case that someone responded to a legendary sorcery being forced in at instant speed by some legal method and another legendary sorcery of the same name also being put onto the stack also by a legal method, what would the outcome be?

Hi,

This is addressed by the Release Notes:

Other than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).

A sorcery having the legendary supertype only carries a casting restriction of “You must control a legendary creature or planeswalker in order to cast this”. There's nothing preventing you from having multiple copies of the same legendary sorcery on the stack at the same time.

Nathan Long
Magic Rules NetRep